RTOS block diagram

 How does Real-time Linux work?

 The general idea of Real-time (RT) Linux is that a small real-time kernel runs beneath Linux, meaning that the real-time kernel has a higher priority than the Linux kernel. 
Real-time tasks are executed by the real-time kernel, and normal Linux programs are allowed to run when no real-time tasks have to be executed. 
Linux can be considered as the idle task of the real-time scheduler. When this idle task runs, it executes its own scheduler and schedules the normal Linux processes. 
Since the real-time kernel has a higher priority, a normal Linux process is preempted when a real-time task becomes ready to run and the real-time task is executed immediately.

 How is the real-time kernel given higher priority than Linux kernel?

 RT Linux uses the flow of interrupts to give the real-time kernel a higher priority than the Linux kernel:

• When an interrupt arrives, it is first given to the real-time kernel, and not to the Linux kernel. But interrupts are stored to give them later to Linux when the real-time kernel has finished.

• As first in row, the real-time kernel can run its real-time tasks driven by these interrupts.

• Only when the real-time kernel is not running anything, the interrupts which were stored are passed on to the Linux kernel.

• As second in row, Linux can schedule its own processes driven by these interrupt.